Pitch-based carbon foam heat sink with phase change material

ABSTRACT

A process for producing a carbon foam heat sink is disclosed which obviates the need for conventional oxidative stabilization. The process employs mesophase or isotropic pitch and a simplified process using a single mold. The foam has a relatively uniform distribution of pore sizes and a highly aligned graphic structure in the struts. The foam material can be made into a composite which is useful in high temperature sandwich panels for both thermal and structural applications. The foam is encased and filled with a phase change material to provide a very efficient heat sink device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a (1) divisional of U.S. patent application Ser. No.09/489,640 filed on Jan. 24, 2000 now U.S. Pat. No. 6,780,505, whichitself is a (2) continuation of U.S. patent application Ser. No.09/093,406, filed on Jun. 8, 1998 now U.S. Pat. No. 6,037,032, which isitself a continuation-in-part of U.S. patent application Ser. No.08/921,875 filed Sep. 2, 1997 now U.S. Pat. No. 6,033,506 and U.S.patent application Ser. No. 08/923,877 filed on Sep. 2, 1997 nowabandoned, and (3) a continuation-in-part of U.S. patent applicationSer. No. 09/337,027 filed Jun. 25, 1999 now U.S. Pat. No. 6,261,485,which itself is a continuation of U.S. patent application Ser. No.08/921,875 filed Sep. 2, 1997, and (4) a continuation-in-part of U.S.patent application Ser. No. 09/136,596 filed Aug. 19, 1998 now U.S. Pat.No. 6,398,343, which itself is a divisional of U.S. patent applicationSer. No. 08/92 1,875 filed Sep. 2, 1997.

This application is a continuation-in-part of an earlier filed U.S.patent application-Ser. No. 08/921,875, filed on Sep. 2, 1997, and U.S.patent application Ser. No. 08/923,877, filed Sep. 2, 1997, both ofwhich are herein incorporated in their entirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The United States Government has rights in this invention pursuant tocontract No. DE-AC05-960R22464 between the United States Department ofEnergy and Lockheed Martin Energy Research Corporation.

BACKGROUND OF THE INVENTION

The present invention relates to porous carbon foam filled with phasechange materials and encased to form a heat sink product, and moreparticularly to a process for producing them.

There are currently many applications that require the storage of largequantities of heat for either cooling or heating an object. Typicallythese applications produce heat so rapidly that normal dissipationthrough cooling fins, natural convection, or radiation cannot dissipatethe heat quickly enough and, thus, the object over heats. To alleviatethis problem, a material with a large specific heat capacity, such as aheat sink, is placed in contact with the object as it heats. During theheating process, heat is transferred to the heat sink from the hotobject, and as the heat sink's temperature rises, it “stores” the beatmore rapidly than can be dissipated to the environment throughconvection. Unfortunately, as the temperature of the heat sink rises theheat flux from the hot object decreases, due to a smaller temperaturedifference between the two objects. Therefore, although this method ofenergy storage can absorb large quantities of heat in some applications,it is not sufficient for all applications

Another method of absorbing heat is through a change of phase of thematerial, rather than a change in temperature. Typically, the phasetransformation of a material absorbs two orders of magnitude greaterthermal energy than the heat capacity of the material. For example, thevaporization of 1 gram of water at 100° C. absorbs 2,439 joules ofenergy, whereas changing the temperature of water from 99° C. to 100° C.only absorbs 4.21 Joules of energy. In other words, raising thetemperature of 579 grams of water from 99° C. to 100° C. absorbs thesame amount of heat as evaporating 1 gram of water at 100° C. The sametrend is found at the melting point of the material. This phenomenon hasbeen utilized in some applications to either absorb or evolve tremendousamounts of energy in situations where heat sinks will not work.

Although a solid block of phase change material has a very largetheoretical capacity to absorb heat, the process is not a rapid onebecause of the difficulties of heat transfer and thus it cannot beutilized in certain applications. However, the utilization of the highthermal conductivity foam will overcome the shortcomings describedabove. If the high conductivity to foam is filled with the phase changematerial, the process can become very rapid. Because of the extremelyhigh conductivity in the struts of the foam, as heat contacts thesurface of the foam, it is rapidly transmitted throughout the foam to avery large surface area of the phase change material. Thus, heat is veryquickly distributed throughout the phase change material, allowing it toabsorb or emit thermal energy extremely quickly without changingtemperature, thus keeping the driving force for heat transfer at itsmaximum.

Heat sinks have been utilized in the aerospace community to absorbenergy in applications such as missiles and aircraft where rapid heatgeneration is found. A material that has a high heat of melting isencased in a graphite or metallic case, typically aluminum, and placedin contact with the object creating the heat. Since most phase changematerials have a low thermal conductivity, the rate of heat transferthrough the material is limited, but this is offset by the high energyabsorbing capability of the phase change. As heat is transmitted throughthe metallic or graphite case to the phase change material, the phasechange material closest to the heat source begins to melt. Since thetemperature of the phase change material does not change until all thematerial melts, the flux from the heat source to the phase changematerial remains relatively constant. However, as the heat continues tomelt more phase change material, more liquid is formed. Unfortunately,the liquid has a much lower thermal conductivity, thus hampering heatflow further. In fact, the overall low thermal conductivity of the solidand liquid phase change materials limits the rate of heat absorptionand, thus, reduces the efficiency of the system.

Recent developments of fiber-reinforced composites, including carbonfoams, have been driven by requirements for improved strength,stiffness, creep resistance, and toughness in structural engineeringmaterials. Carbon fibers have led to significant advancements in theseproperties in composites of various polymeric, metal, and ceramicmatrices.

However, current applications of carbon fibers have evolved fromstructural reinforcement to thermal management in application rangingfrom high-density electronic modules to communication satellites. Thishas stimulated research into novel reinforcements and compositeprocessing methods. High thermal conductivity, low weight, and lowcoefficient of thermal expansion are the primary concerns in thermalmanagement applications. See Shih, Wei, “Development of Carbon-CarbonComposites for Electronic Thermal Management Applications,” IDAWorkshop, May 3–5, 1994, supported by AF Wright Laboratory underContract Number F33615-93-C-2363 and AR Phillips Laboratory ContractNumber F29601-93C-0165 and Engle, G. B., “High Thermal Conductivity C/CComposites for Thermal Management,” IDA Workshop, May 3–5, 1994,supported by AF Wright Laboratory under Contract F33615-93-C-2363 and ARPhillips Laboratory Contract Number F29601-93-C-0165. Such applicationsare striving towards a sandwich type approach in which a low-densitystructural core material (i.e. honeycomb or foam) is sandwiched betweena high thermal conductivity facesheet. Structural cores are limited tolow density materials to ensure that the weight limits are not exceeded.Unfortunately, carbon foams and carbon honeycomb materials are the onlyavailable materials for use in high temperature applications (>1600°C.). High thermal conductivity carbon honeycomb materials are extremelyexpensive to manufacture compared to low conductivity honeycombs,therefore, a performance penalty is paid for low cost materials. Highconductivity carbon foams are also more expensive to manufacture thanlow conductivity carbon foams, in part, due to the starting materials.

In order to produce high stiffness and high conductivity carbon foams,invariably, a pitch must be used as the precursor. This is because pitchis the only precursor which forms a highly aligned graphitic structurewhich is a requirement for high conductivity. Typical processes utilizea blowing technique to produce a foam of the pitch precursor in whichthe pitch is melted and passed from a high pressure region to a lowpressure region. Thermodynamically, this produces a “Flash,” therebycausing the low molecular weight compounds in the pitch to vaporize (thepitch boils), resulting in a pitch foam. See Hagar, Joseph W. and Max L.Lake, “Novel Hybrid Composites Based on Carbon Foams,” Mat. Res. Soc.Symp ., Materials Research Society, 270:29–34 (1992); Hagar, Joseph W.and Max L. Lake, “Formulation of a Mathematical Process Model ProcessModel for the Foaming of a Mesophase Carbon Precursor,” Mat. Res. Soc.Symp., Materials Research Society, 270:35–40 (1992); Gibson, L. J. andM. F. Ashby, Cellular Solids: Structures & Properties, Pergamon Press,New York (1988); Gibson, L. J., Mat. Sci. and Eng A110, 1 (1989);Knippenberg and B. Lersmacher, Phillips Tech. Rev., 36(4), (1976); andBonzom, A., P. Crepaux and E. J. Moutard, U.S. Pat. No. 4,276,246,(1981). Then, the pitch-foam must be oxidatively stabilized by heatingin air (or oxygen) for many hours, thereby, crosslinking the structureand “stabilizing” the pitch so it does not melt during carbonization.See Hagar, Joseph W. and Max L. Lake, “Formulation of a MathematicalProcess Model Process Model for the Foaming of a Mesophase CarbonPrecursor, Mat. Res. Soc. Symp., Materials Research to Society,270:35–40 (1992); and White, J. L., and P.M. Shaeffer. Carbon, 27:697(1989). This is a time consuming step and can be an expensive stepdepending on the part size and equipment required. The “stabilized” oroxidized pitch is then carbonized in an inert atmosphere to temperaturesas high as 1100° C. Then, graphitization is performed at temperatures ashigh as 3000° C. to produce a high thermal conductivity graphiticstructure, resulting in a stiff and very thermally conductive foam.

Other techniques utilize a polymeric precursor, such as phenolic,urethane, or blends of these with pitch. See Hagar, Joseph W. and Max L.Lake, “Idealized Strut Geometries for Open-Celled Foams,” Mat. Res. Soc.Symp., Materials Research Society, 270:41–46 (1992); Aubert, J. W., MRSSymposium Proceedings, 207:117–127 (1990); Cowlard, F. C. and J. C.Lewis, J. of Mat. Sci., 2:507–512 (1967); and Noda, T., Inagaki and S.Yamada, J. of Non-Crystalline Solids, 1:285–302, (1969). High pressureis applied and the sample is heated. At a specified temperature, thepressure is released, thus causing the liquid to foam as volatilecompounds are released. The polymeric precursors are cured and thencarbonized without a stabilization step. However, these precursorsproduce a “glassy” or vitreous carbon which does not exhibit graphiticstructure and, thus, has low thermal conductivity and Low stiffness. SeeHagar, Joseph W. and Max L. Lake, “Idealized Strut Geometries forOpen-Celled Foams,” Mat. Res. Soc. Symp., Materials Research Society,270:41–46 (1992).

In either case, once the foam is formed, it is then bonded in a separatestep to the facesheet used in the composite. This can be an expensivestep in the utilization of the foam.

The extraordinary mechanical properties of commercial carbon fibers aredue to the unique graphitic morphology of the extruded filaments. SeeEdie, D. D., “Pitch and Mesophase Fibers,” in Carbon Fibers, Filamentsand Composites, Figueiredo (editor), Kluwer Academic Publishers, Boston,pp. 43–72 (1990). Contemporary advanced structural composites exploitthese properties by creating a disconnected network of graphiticfilaments held together by an appropriate matrix. Carbon foam derivedfrom a pitch precursor can be considered to be an interconnected networkof graphitic ligaments, or struts, as shown in FIG. 1. As suchinterconnected networks, they represent a potential alternative asreinforcement in structural composite materials.

The process of this invention overcomes current manufacturinglimitations by avoiding a “blowing” or “pressure release” technique toproduce the foam. Furthermore, an oxidation stabilization step is notrequired, as in other methods used to produce pitch based carbon foamswith a highly aligned graphitic structure. This process is less timeconsuming, and therefore, will be lower in cost and easier to fabricate.The foam can be produced with an integrated sheet of high thermalconductivity carbon on the surface of the foam, thereby producing acarbon foam with a smooth sheet on the surface to improve heat transfer.

SUMMARY OF THE INVENTION

An object of the present invention is production of encased high thermalconductivity porous carbon foam filled with a phase change materialwherein tremendous amounts of thermal energy are stored and emitted veryrapidly. The porous foam is filled with a phase change material (PCM) ata temperature close to the operating temperature of the device. As heatis added to the surface, from a heat source such as a computer chip,friction due to re-entry through the atmosphere, or radiation such assunlight, it is transmitted rapidly and uniformly throughout the foamand then to the phase change material. As the material changes phase, itabsorbs orders of magnitude more energy than non-PCM material due totransfer of the latent heat of fusion or vaporization. Conversely, thefilled foam can be utilized to emit energy rapidly when placed incontact with a cold object.

Non-limiting embodiments disclosed herein are a device to rapidly thawfrozen foods or freeze thawed foods, a design to prevent overheating ofsatellites or store thermal energy as they experience cyclic heatingduring orbit, and a design to cool leading edges during hypersonicflight or re-entry from space.

Another object of the present invention is to provide carbon foam and acomposite from a mesophase or isotropic pitch such as synthetic,petroleum or coal-tar based pitch. Another object is to provide a carbonfoam and a composite from pitch which does not require an oxidativestabilization step.

These and other objectives are accomplished by a method of producingcarbon foam heat sink wherein an appropriate mold shape is selected andpreferably an appropriate mold release agent is applied to walls of themold. Pitch is introduced to an appropriate level in the mold, and themold is purged of air by applying a vacuum, for example. Alternatively,an inert fluid could be employed. The pitch is heated to a temperaturesufficient to coalesce the pitch into a liquid which preferably is ofabout 50° C. to about 100° C. above the softening point of the pitch.The vacuum is released and an inert fluid applied at a static pressureup to about 1000 psi. The pitch is heated to a temperature sufficient tocause gases to evolve and foam the pitch. The pitch is further heated toa temperature sufficient to coke the pitch and the pitch is cooled toroom temperature with a simultaneous and gradual release of pressure.The foam is then filled with a phase change material and encased toproduce an efficient heat storage product.

In another aspect, the previously described steps are employed in a moldcomposed of a material such that the molten pitch does not adhere to thesurface of the mold.

In yet another aspect, the objectives are accomplished by the carbonfoam product produced by the methods disclosed herein including a foamproduct with a smooth integral facesheet.

In still another aspect a carbon foam composite product is produced byadhering facesheets to the carbon foam produced by the process of thisinvention.

FIG. 1 is section cut of a heat sink device for thawing food usingacetic acid as the phase change material.

FIG. 2 is a section cut of a heat sink to prevent overheating ofsatellites during cyclic orbits.

FIG. 3 is a section cut of a heat sink used on the leading edge of ashuttle orbiter.

FIG. 4 is a micrograph illustrating typical carbon foam withinterconnected carbon ligaments and open porosity.

FIGS. 5–9 are micrographs of pitch-derived carbon foam graphitized at2500° C. and at various magnifications.

FIG. 10 is a SEM micrograph of carbon foam produced by the process ofthis invention.

FIG. 11 is a chart illustrating cumulative intrusion volume versus porediameter.

FIG. 12 is a chart illustrating log differential intrusion volume versuspore diameter.

FIG. 13 is a graph illustrating the temperatures at which volatiles aregiven off from raw pitch.

FIG. 14 is an X-ray analysis of the graphitized foam produced by theprocess of this invention.

FIGS. 15 A–C are photographs illustrating foam produced with aluminumcrucibles and the smooth structure or face sheet that develops. FIG. 16Ais a schematic view illustrating the production of a carbon foamcomposite made in accordance with this invention.

FIG. 16B is a perspective view of the carbon foam composite of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

In order to illustrate the carbon foam heat sink product of thisinvention, the following examples are set forth. They are not intendedto limit the invention in any way.

EXAMPLE 1 Device for Thawing Food

Acetic acid has a heat of melting of 45 J/g at a melting point of 11° C.The heat of melting of food, primarily ice, is roughly 79 J/g at 0° C.Therefore, take a block of foam and fill it with liquid acetic acid atroom temp. The foam will be encased in a box made from an insulatingpolymer such as polyethylene on all sides except the top. The top of thefoam/acetic acid block will be capped with a high thermal conductivityaluminum plate that snaps into place thus sealing the foam/acetic acidinside the polymer case (illustrated in FIG. 1). If the foam block is10-in.×15-in.×0.5-in. thick, the mass of foam is 614 grams. The mass ofacetic acid that fills the foam is roughly 921 grams. Therefore, when apiece of frozen meat is placed in contact with the top of the aluminumblock, the foam will coot to the freezing point of the acetic acid (11°C.). At this point, the heat given off from the acetic acid as itfreezes (it also remains at 11° C.) will be equivalent to 49 KJ. Thisheat is rapidly transferred to the frozen meat as it thaws (it alsoremains at 0° C.). This amount of heat is sufficient to melt roughly 500grams (1 lb.) of meat.

EXAMPLE 2 Heat Sink To Prevent Overheating of Satellites During CyclicOrbits.

Produce a carbon-carbon composite with the foam in which the foam is acore material with carbon-carbon face sheets (FIG. 2). Fill the foamcore with a suitable phase change material, such as a paraffin wax, thatmelts around the maximum operating temperature of the satellitecomponents. One method to perform this would be to drill a hole in onesurface of the carbon-carbon face sheets and vacuum fill the phasechange material in the liquid state into the porous foam. Once filled,the sample can be cooled (the phase change material solidifies) and thehole can be plugged with an epoxy or screw-type cap. The epoxy and anyother sealant must be able to withstand the operating temperature of theapplication. The foam-core composite will to then be mounted on the sideof the satellite that is exposed to the sun during orbit. As thesatellite orbits the earth and is exposed to the sun, the radiant energyfrom the sun will begin to heat the composite panel to the melting pointof the phase change material. At this point, the panel will not increasein temperature as the phase change material melts. The amount of radiantenergy the panel can absorb will be dependent on the thickness and outerdimensions of the panel. This can be easily calculated and designedthrough knowledge of the orbit times of the satellite such that thematerial never completely melts and, thus, never exceeds the melttemperature. Then, when the satellite leaves the view of the sun, itwill begin to radiate heat to space and the phase change material willbegin to freeze. The cycle will repeat itself once the satellite comesinto view of the sun once again.

EXAMPLE 3 Heat Sink for Leading Edges

Currently, the shuttle orbiter experiences extreme heats during reentry.Specifically, the leading edges of the craft can reach 1800° C. and thebelly of the craft can reach temperatures as high as 1200° C. If a foamcore composite panel is placed at the surface of the leading edges andat the surface of the belly (FIG. 3), it would be able to absorb enoughenergy to dramatically reduce the maximum temperature of the hot areas.This also would permit a faster re-entry or (steeper glide slope) andmaintain the current maximum temperatures. In this case the phase changematerial would most likely be an alloy, e.g. germanium-silicon, whichmelts around 800–900C. and does not vaporize until much higher than themaximum temperature of the craft.

For example, Germanium has a heat of formation (heat of melting) of 488J/g. This would require 1.0 Kg of Germanium to reduce the temperature of1 Kg of existing carbon/carbon heat-shield by 668° C. In other words, ifthe existing carbon-carbon were replaced pound-for-pound with germaniumfilled foam, the maximum temperature of the heat shield would only beabout 1131° C. instead of about 1800° C. during re-entry, depending onthe duration of thermal loading.

EXAMPLE 4

Pitch powder, granules, or pellets are placed in a mold with the desiredfinal shape of the foam. These pitch materials can be solvated ifdesired. In this example, Mitsubishi ARA-24 mesophase pitch wasutilized. A proper mold release agent or film is applied to the sides ofthe mold to allow removal of the part. In this case, Boron Nitride sprayand Dry Graphite Lubricant were separately used as a mold release agent.If the mold is made from pure aluminum, no mold release agent ISnecessary since the molten pitch does not adhere to the aluminum and,thus, will not stick to the mold. Similar mold materials may be foundthat the pitch does not adhere and, thus, they will not need moldrelease. The sample is evacuated to less than 1 torr and then heated toa temperature approximately 50 to 100° C. above the softening point. Inthis case where Mitsubishi ARA24 mesophase pitch was used, 300° C. wassufficient. At this point, the vacuum is released to a nitrogen blanketand then a pressure of up to 1000 psi is applied. The temperature of thesystem is then raised to 800° C., or a temperature sufficient to cokethe pitch which is 500° C. to 1000° C. This is performed at a rate of nogreater than 5° C./min. and preferably at about 2° C./min. Thetemperature is held for at least 15 minutes to achieve an assured soakand then the furnace power is turned off and cooled to room temperature.Preferably the foam was cooled at a rate of approximately 1.5° C./min.with release of pressure at a rate of approximately 2 psi/min. Finalfoam temperatures for three product runs were 500° C., 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns to 2500° C. and 2800° C. (graphitized) in Argon.

Carbon foam produced with this technique was examined withphotomicrography, scanning electron microscopy (SEM), X-ray analysis,and mercury porisimetry. As can be seen in the FIGS. 5–10, theisochromatic regions under cross-polarized light indicate that thestruts of the foam are completely graphitic. That is, all of the pitchwas converted to graphite and aligned along the axis of the struts.These struts are also similar in size and are interconnected throughoutthe foam. This would indicate that the foam would have high stiffnessand good strength. As seen in FIG. 10 by the SEM micrograph of the foam,the foam is open cellular meaning that the porosity is not closed. FIGS.11 and 12 are results of the mercury porisimetry tests. These testsindicate that the pore sizes are in the range of 90–200 microns.

A thermogravimetric study of the raw pitch was performed to determinethe temperature at which the volatiles are evolved. As can be seen inFIG. 14, the pitch loses nearly 20% of its mass fairly rapidly in thetemperature range between about 420° C. and about 480° C. Although thiswas performed at atmospheric pressure, the addition of 1000 psi pressurewill not shift this effect significantly. Therefore, while the pressureis at 1000 psi, gases rapidly evolved during heating through thetemperature range of 420° C. to 480° C. The gases produce a foamingeffect (like boiling) on the molten pitch. As the temperature isincreased further to temperatures to ranging from 500° C. to 1000° C.(depending on the specific pitch), the foamed pitch becomes coked (orrigid), thus producing a solid foam derived from pitch. Hence, thefoaming has occurred before the release of pressure and, therefore, thisprocess is very different from previous art.

Samples from the foam were machined into specimens for measuring thethermal conductivity. The bulk thermal conductivity ranged from 58 W/m·Kto 106 W/m·K. The average density of the samples was 0.53 g/cm³. Whenweight is taken into account, the specific thermal conductivity of thepitch derived from foam is over 4 times greater than that of copper.Further derivations can be utilized to estimate the thermal conductivityof the struts themselves to be nearly 700 W/m·K. This is comparable tohigh thermal conductivity carbon fibers produced from this same ARA24mesophase pitch.

X-ray analysis of the foam was performed to determine the crystallinestructure of the material. The x-ray results are shown in FIG. 14. Fromthis data, the graphene layer spacing (d₀₀₂) was determined to be 0.336nm. The coherence length (L_(a,100)) was determined to be 203.3 nm andthe stacking height was determined to be 442.3 nm.

The compression strength of the samples were measured to be 3.4 MPa andthe compression modulus was measured to be 73.4 MPa. The foam sample waseasily machined and could be handled readily without fear of damage,indicating good strength.

It is important to note that when this pitch is heated in a similarmanner, but only under atmospheric pressure, the pitch foamsdramatically more than when under pressure. In fact, the resulting foamis so fragile that it could not even be handled to perform tests.Molding, under pressure serves to limit the growth of the cells andproduces a usable material.

EXAMPLE 5

An alternative to the method of Example 4 is to utilize a mold made fromaluminum. In this case two molds were used, an aluminum weighing dishand a sectioned soda can. The same process as set forth in Example 4 isemployed except that the final coking temperature was only 630° C., soas to prevent the aluminum from melting.

FIGS. 15 A–C illustrate the ability to utilized complex shaped molds forproducing complex shaped foam. In one case, shown in FIG. 15A, the topof a soda can was removed and the remaining can used as a mold. Norelease agent was utilized. Note that the shape of the resulting partconforms to the shape of the soda can, even after graphitization to2800° C. This demonstrates the dimensional stability of the foam and theability to produce near net shaped parts.

In the second case, as shown in FIGS. 15 B and C employing an aluminumweight dish, a very smooth surface was formed on the surface contactingthe aluminum. This is directly attributable to the fact that the moltenpitch does not adhere to the surface of the aluminum. This would allowone to produce complex shaped parts with smooth surfaces so as toimprove contact area for bonding or improving heat transfer. This smoothsurface will act as a face sheet and, thus, a foam-core composite can befabricated in-situ with the fabrication of the face sheet. Since it isfabricated together and an integral material no interface joints result,thermal stresses will be less, resulting in a stronger material.

The following examples illustrate the production of a composite materialemploying the foam of this invention.

EXAMPLE 6

Pitch derived carbon foam was produced with the method described inExample 4. Referring to FIG. 16A the carbon foam 10 was then machinedinto a block 2″×2″ 1/2″. Two pieces 12 and 14 of a prepeg comprised ofHercules AS4 carbon fibers and ICI Fibirite Polyetheretherkeytonethermoplastic resin also of 2″×2″× 1/2″ size were placed on the top andbottom of the foam sample, and all was placed in a matched graphite mold16 for compression by graphite plunger 18. The composite sample washeated under an applied pressure of 100 psi to a temperature of 380° C.at a rate of 5° C./min. The composite was then heated under a pressureof 100 psi to a temperature of 650° C. The foam core sandwich panelgenerally 20 was then removed from the mold and carbonized undernitrogen to 1050° C. and then graphitized to 2800° C., resulting in afoam with carbon-carbon facesheets bonded to the surface. The compositegenerally 30 is shown in FIG. 16B.

EXAMPLE 7

Pitch derived carbon foam was produced with the method described inExample 4. It was then machined into a block 2″×2″× 1/2″. Two pieces ofcarbon-carbon material, 2″×2″× 1/2″, were coated lightly with a mixtureof 50% ethanol, 50% phenolic Durez© Resin available from OccidentalChemical Co. The foam block and carbon-carbon material were positionedtogether and placed in a mold as indicated in Example 6. The sample washeated to a temperature of 150° C. at a rate of 5° C./min and soaked attemperature for 14 hours. The sample was then carbonized under nitrogento 1050° C. and then graphitized to 2800° C., resulting in a foam withcarbon-carbon facesheets bonded to the surface. This is also showngenerally at 30 in FIG. 16B.

EXAMPLE 8

Pitch derived carbon foam was produced with the method described inExample 4. The foam sample was then densified with carbon by the methodof chemical vapor infiltration for 100 hours. The density increased to1.4 g/cm³, the flexural strength was 19.5 MPa and the flexural moduluswas 2300 MPa. The thermal conductivity of the raw foam was 58 W/m·K andthe thermal conductivity of the densified foam was 94 W/m·K.

EXAMPLE 9

Pitch derived carbon foam was produced with the method described inExample 4. The foam sample was then densified with epoxy by the methodof vacuum impregnation. The epoxy was cured at 150° C. for 5 hours. Thedensity increased to 1.37 g/cm³ and the flexural strength was measuredto be 19.3 MPa.

Other possible embodiments may include materials, such as metals,ceramics, plastics, or fiber-reinforced plastics bonded to the surfaceof the foam of this invention to produce a foam core composite materialwith acceptable properties. Additional possible embodiments includeceramics, glass, or other materials impregnated into the foam fordensification.

Based on the data taken to date from the carbon foam material, severalobservations can be made outlining important features of the inventionthat include:

1. Pitch-based carbon foam can be produced without an oxidativestabilization step, thus saving time and costs.

2. High graphitic alignment in the struts of the foam is achieved upongraphitization to 2500° C., and thus high thermal conductivity andstiffness will be exhibited by the foam, making them suitable as a corematerial for thermal-applications.

3. High compressive strengths should be achieved with mesophasepitch-based carbon foams, making them suitable as a core material forstructural applications.

4. Foam core composites can be fabricated at the same time as the foamis generated, thus saving time and costs.

5. Rigid monolithic preforms can be made with significant open porositysuitable for densification by the Chemical Vapor Infiltration method ofceramic and carbon infiltrants.

6. Rigid monolithic preforms can be made with significant open porositysuitable for activation, producing a monolithic activated carbon.

7. It is obvious that by varying the pressure applied, the size of thebubbles formed during the foaming will change and, thus, the density,strength, and other properties can be affected.

The following alternative procedures and products can also be effectedby the process of this invention:

1. Fabrication of preforms with complex shapes for densification by CVIor Melt Impregnation.

2. Activated carbon monoliths with high thermal conductivity.

3. Optical absorbent.

4. Low density heating elements.

5. Firewall Material

6. Low secondary electron emission targets for high-energy physicsapplications.

The present invention provides for the manufacture of pitch-based carbonfoam heat sink for structural and thermal composites. The processinvolves the fabrication of a graphitic foam from a mesophase orisotropic pitch which can be synthetic, petroleum, or coal-tar based. Ablend of these pitches can also be employed. The simplified processutilizes a high pressure high temperature furnace and thereby, does notrequire and oxidative stabilization step. The foam has a relativelyuniform distribution of pore sizes (−100 microns), very little closedporosity, and density of approximately 0.53 g/cm³. The mesophase pitchis stretched along the struts of the foam structure and thereby producesa highly aligned graphitic structure in the struts. These struts willexhibit thermal conductivities and stiffness similar to the veryexpensive high performance carbon fibers (such as P-120 and K1100).Thus, the foam will exhibit high stiffness and thermal conductivity at avery low density (−0.5 g/cc). This foam can be formed in place as a corematerial for high temperature sandwich panels for both thermal a,idstructural applications, thus reducing fabrication time. By utilizing anisotropic pitch, the resulting foam can be easily activated to produce ahigh surface area activated carbon. The activated carbon foam will notexperience the problems associated with granules such as attrition,channeling, and large pressure drops.

1. A temperature control apparatus for aiding in maintaining thetemperature of an object in contact therewith below 1800° C. comprisingan encased carbon foam containing in at least some of its pores a phasechange material that melts at an elevated temperature above about 800°C. but does not vaporize below 1800° C.
 2. An apparatus as defined inclaim 1 wherein said phase change material melts between about 800° C.and 900° C.
 3. An apparatus as defined in claim 1 wherein said phasechange material comprises germanium.
 4. An apparatus as defined in claim1 wherein said phase change material comprises germanium-silicon.
 5. Atemperature control apparatus for aiding in maintaining the temperatureof an object in contact therewith below 1200° C. comprising an encasedcarbon foam containing in at least some of its pores a phase changematerial that melts at an elevated temperature above about 800° C. butdoes not vaporize below 1200° C.
 6. An apparatus as defined in claim 5wherein said phase change material melts between about 800° C. and 900°C.
 7. An apparatus as defined in claim 5 wherein said phase changematerial comprises germanium.
 8. An apparatus as defined in claim 5wherein said phase change material comprises germanium-silicon.